1. Field of the Invention
This invention relates generally to brake control systems, and more specifically to an improved vehicle brake control system.
2. Description of the Prior Art
U.S. Pat. No. 3,691,524 relates to a tire inflation monitoring system for monitoring the angular movement of the wheels of a vehicle. By counting the revolutions on each of the wheels and comparing the count with the count from another wheel, it is possible to determine whether or not each wheel has the same diameter. An indicating means is provided for signalling an occupant of the vehicle when a tire thereon is underinflated. A disadvantage of this prior art monitoring system is its lack of sensitivity which precludes its use for operating a meter or indicating differences in wheel revolutions during braking.
U.S. Pat. No. 2,522,139 relates to a frequency responsive system for comparing two alternating current frequencies and creating a third frequency in relation to the difference in the two frequencies to indicate the sense and magnitude of this difference. A disadvantage of this frequency responsive system is that the compared frequencies involved are high and the difference frequency low in relation thereto so that this system is not readily usable in a monitoring system of the type disclosed in U.S. Pat. No. 3,691,524.
U.S. Pat. No. 3,797,893 relates to a brake force control system for vehicles in which a sensor is coordinated with each wheel for detecting its rotational condition. The signals from the sensors actuate inlet and outlet valves when they exceed or drop below certain threshold values so that the brake pressure either increases, remains constant, or decreases. In vehicles with a high center of gravity and especially with a short wheel base, an additional logic circuit connection is provided which decreases the pressure at the front wheel brakes if a signal symbolizing the road traction of the rear wheels does not arrive within a certain time delay.
U.S. Pat. Nos. 3,260,555, 3,482,887 and 3,756,663 are exemplary of anti-skid brake systems having electrical means for sensing the rotational speed of individual wheels. An electrical signal is obtained as a result of a variation in the signal in relation to a predetermined characteristic. The signal is used to automatically operate means for controlling and releasing the braking force on any wheel or number of wheels revolving slower than a desired speed. Although the latter patents all operate satisfactorily for preventing the wheels of a vehicle to skid, none of them are capable, among other things, of determining the prebraking rotational speed ratio of a pair of wheels and by controlling the brake force applied to the wheels maintaining the braking rotational speed ratio substantially equal to the prebraking rotational speed ratio.